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MG MGB Technical - Electrical Issue

I am in the process of resurrecting my dads 1978 mgb gt. Firstly let me just give you background on the car he has owned it since new and it has been completely unmolested everything is original. it has been sat untouched in his garage for 20 years. I have got the engine running so have moved on to the electrics. The following is working:

No temp gauge
No fuel gauge

No indicators
No Hazard
No Horn
No heater blower

I am hoping there is a simple explanation for all this ie a fuse? I have checked the ones in the engine bay but are there any more that I am missing?

Any help would be appreciated


C Carter

Chris. First step is to get a wiring diagram for the car, then have it expanded about 300% and laminated. I have been using mine for 15 years now and it is still in good condition. With the diagram, you can take one of the erasable markers and draw out the bad circuits to see if them come together and, if so, where.

One of the easiest test instruments to use is the test light which allows you to quickly check for current flow by simply grounding the wire from the test light to the engine block, then touching the probe to a terminal to see if the light illuminates. If it does, you have current flow and can keep checking further in the circuit to see where the current flow stops.

As your first check, look at the green wire circuit which is fed by the third fuse from the top. Check for power into the fuse, power at the clip holding the fuse on the input (front--towards front of car) terminal, then on the clip holding the fuse, then on the front metal cap of the fuse, the rear metal cap of the fuse, the rear clip holding the fuse, then the rear terminal where the green wires are attached. This system quickly tests the terminals, the connections of the terminals to the clips, connection of the clips to the fuse, etc. Often, the inside of the clips is simply corroded and they need to be cleaned for proper connections with the fuse to be made.

Les Bengtson

Are the items you list the ONLY things not working? Logically, I would assume your tach is not working either..?

I agree that everything points towards the green circuit fuse, but there are two circuits off this fuse and a handy way of checking if it is the fuse is if the items you list plus brake/reversing lights are all not working. It would be a good idea as Les suggested to clean up the fuses anyway as they are of the most antiquated design imaginable.

There is on your car another inline fuse to a green circuit. BL could not design a car to save their lives (witness their demise) and when they added new circuitry, rather than expand the fuse box, they put an inline fuse on the circuit. Look for it on the inner wing where a green wire joins a white wire. This fuse protects indicators and hazards but NOT the temp, fuel gauges!

Finally the horn is not related to any of the others on your list - it is on a separate fuse (top one from memory). Now, here, check that you have interior lights and trunk/hatch courtesy lights. If you do, then it is not fuse but more likely the contacts in the horn button.

It sounds to me as if there may be a few problems. But do not panic, they are all simple! Start with the green circuit main fuse and see if that brings back anything. Then look for the in-line fuse joining a green and white wire.
Hal Adams

Hazards shouldn't be on the green circuit, as they are supposed to work without the ignition being on.
Dave O'Neill2

Had the same problem, same year car, corroded fusebox ....problem solved.
M F L Sherrit

Apologies - I was too quick - indicators, heater and tacho (possibly rear window demister as well - I cant remember) only are protected by the in-line fuse. For the hazards, look for another in-line fuse joining two brown wires, from memory In the same region as the other one.
Hal Adams

Ok been looking at the car again today. Tacho is fine and fully working as is the oil pressure. I have looked at the wring diagram and it shows that the fuel gauge and temp sensor seem to go through a voltage stabilizer so could this be my problem? As for the horn and indicators I will have to logically work through the problem. I have cleaned all the fuses and it has not made a difference but in all honesty I didnt think it would as the ignition circuit is fine. I am thinking that it is more likely something to do with the actually right hand stalk as the windscreen wipers work fine? How would I go about cleaning the contacts on the stalk?
C Carter

More than once in the past, I have been fooled by fuse cartridges that appeared to be intact, but when tested with an ohmmeter, were found to be open circuited. Another trap is one of the in line fuses has a different coloured wire at each end.(Can't remember which). But it's easy to interconnect them wrongly to another fuse holder. Barrie E
B Egerton

fuel gauge and temp sensor do go via a voltage stabiliser and you are right to check it. However, It's a very reliable unit and rarely fails so I think I would be looking elsewhere. I you have all the other "green" wire feb systems eg wiper, fan etc then perhaps the staberliser is at fault.
The wiper stalk is easy enough to remove and clean! the contacts on the wiper are usually just copper rivet heads with a spring stalk sliding over them. Clean with degreaser and fine wet and dry and then coat with Vaseline. But before you go and strip it down I would disconnect it and, using a short piece of wire, connect across the connector therefore bypassing the switch to see if the rest of the circuit is okay.

Best of

M McAndrew

The stabiliser could also be an issue, yes. After cleaning the fuses, this is one of the next things in the logical list.

My only concern is that you say there is no heater blower yet you have the tacho - both are on the same circuit, so it could be now that you have another problem with the heater...

Back to the gauges. Can you locate the stabiliser? It is up behind the dash on the bulkhead, a small metal box with a green wire on one terminal and a two tone green on the other terminal. Do you have a multimeter? if so, check that you have voltage on the terminal with the two tone green wire. It should be 10V plus or minus a volt or so.

If you have tacho and no indicators, then it is not a fuse issue. But before you go pulling the stalk to pieces, pull out the hazard light switch. On the back you will see three green wires - two on one terminal and one on another. Join these three wires together (or bridge the two terminals). Only if after this the indicators do not work, look at the flasher unit and then the stalk.

For the horn - do you have courtesy lights?
Hal Adams

Another brainstorm. Just remembered when I bought a'78 GT back in '85. It had a few electrical issues which were caused by the rocker switches being full of green crap. A simple cleanout cured the problems. Since your car has laid idle for 20 years, I think the switches are worth checking as a matter of course. Also,the hot wire for the indicators is via the Hazard switch. Barrie E
B Egerton

Worth Checking I will have a look at the rocker switches
C Carter

I do urge you to follow a path checking out the simple possibilities before you go breaking open switches first. They are fiddly things at best.

There is only one rocker switch that might be suspect - the hazard switch, but first bridge pins 7 and 8 (the three green wires) as I suggested above. If the indicators still do not work, then put a known 12V supply onto the single green wire on pin 8. Only if this gets them working, break open the switch, or better, replace.
Hal Adams

A good way of cleaning electrics is Servisol switch cleaning spray. A blast inside switches etc. will do them the world of good.

Points of failure for electrics on a 78 car are the two black cylindrical relays near the fuse box in the engine bay which corrode. By carefully easing the top off these from around the base where the wires plug in you can get inside and clean them out. When reassembling use mastic sealer around the base and they will be good for many years. I sold my 78 GT about the same time your Dad's went off the road and both relays had failed & been repaired.

Another other failure I had, quite incredibly, was that the fuel pump power lead in the loom near the pump corroded through and I had to run another wire to it.

I think I had one of the rocker switches under the fresh air vents fail. These are standard BL parts found on the Jaguar XJS etc.

The only other electrical thing I can remember doing was to replace the brushes in the electric engine cooling fan. These days replacement with a Kenlowe is probably a good idea - but so is originality so making do and mending is also a good policy!

John Prewer

This thread was discussed between 29/09/2012 and 02/10/2012

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